The active implantable medical devices for use with the present invention include those devices having circuits for detecting the cardiac activity, i.e. detecting (or sensing) the spontaneous depolarization of the myocardium, as well as circuits for stimulating the myocardium, which circuits are in themselves well-known and of different specific constructions.
It is important to be able to sense the signal resulting from the myocardial depolarization as soon as possible after the stimulation, in order to detect as accurately and quickly as possible a depolarization wave revealing an activity of the myocardial cells. This makes it possible to carry out, for example, very precise algorithms for the control of the heartbeat rate to enable or obtain a more physiological behavior of the prosthesis (the implanted device). This also allows for a reduction of energy consumption by delivering only stimulation that is suitable for the given condition. This detection is also used to control the operation of certain cardiac rate control algorithms such as the so-called fallback and smoothing algorithms. In addition, the detection of the spontaneous ventricular rate, in particular the analysis of its stability, are important parameters in certain implantable defibrillators for the release of a shock therapy.
The checking of the effectiveness of the stimulation is an important feature to maintain the implanted device in its optimal operating range. This is done by performing what is called a “capture test” that measures an “evoked potential”, i.e., the potential of the depolarization wave induced by a stimulation of the cavity being monitored, using an intracardiac lead (also called a probe) having an electrode in contact with the myocardium. The evoked potential signal maybe distorted, however, by disturbances that are related to the behavior of the sensing circuit amplifiers just after stimulation. A first type of disturbance comes from the discharge of the electric charges at the electrode/myocardium interface. To eliminate the effects of this disturbance, one envisages a period known as a “refractory period” during which a disconnection (also referred to as a “blanking”) of the sensing circuits is operated, typically for a length of time of about 13 ms. By comparison, a stimulation pulse has a maximum duration of about 1 ms. This blanking period typically is an “absolute” blanking period in which no signals are detected, and is more typically followed by a period of waiting or “listening” for an evoked response, of a typical duration of about 50 ms. The signal delivered by the sensing circuits during this listening period is however disturbed by another factor, specific to the sensing detection amplifier, because of a phenomenon of “rebound” of the amplifier at the time when it is reconnected to the sensing electrode at the conclusion of the blanking period.
This potential specific to the amplifier is hereafter referred to as a “response potential.”The signal delivered by the sensing circuit amplifier will thus include, if it is present, the evoked potential resulting from the depolarization of the cavity, on which a specific response potential from the amplifier will be superimposed. The presence and the level of the amplifier response potential will be primarily independent of the presence or the absence of a depolarization; it is thus likely to disturb the capture test, particularly when the depolarization wave has a low amplitude, as is the case, for example, with an atrial depolarization wave because of the small muscular mass of the atrium.